In the present work, the results of the neutron scattering, the ball-drop method, DSC, and the intrinsic viscosity on atactic polystyrene (aPS)/carbon disulfide (CS_2) system were presented to reveal the mechanism of the sol-gel transition. The pregel is formed by the interchain association among aPS chains even in the sol. Through the analysis of the viscosity in the limits of both infinite dilution and high molecular weight, the aPS chain behaves as a kind of globular coil, which is induced by the strong interchain association. The critical scattering exponents along the sol-gel transition curve change from 3.3 to 2.0, as the concentration increases. The result suggests that the cross-linking region has a soft structure, i.e., it behaves like a fluid nodule. The results on the wide-angle neutron scattering suggest that the gel structure largely depends on the preparation temperature of the gel. The soft crosslinking region originates from the participation of the solvent molecule in the formation of the crosslinks. As a conclusion, present physical gel may be the phase separating gel with soft corsslinks.